In a DS-CDMA mobile cellular communications system, two significant channel impairments are co-channel interference from other users and fading of the signal including Doppler frequency shifts when the mobile station is moving. Modulation and coding schemes used in the system enable a desirably low error probability to be achieved for a given SIR (signal-to-interference ratio) corresponding to a certain capacity, i.e. number of users or mobile stations, of the system. It is known that coherent detection techniques are preferable to non-coherent techniques for optimizing SIR and hence providing an optimal system capacity, because coherent detection techniques enable the different diversity signals to be combined with one another in phase.
It is known to provide a communications system in which coherent detection is achieved by combining diversity signals after aligning them in phase using PLL (phase locked loop) techniques. However, PLL techniques may not function properly in fading environments, such as in cellular communications systems, due to an increased cycle skipping rate.
It is also known for example in an IS-95 system to provide a pilot signal in the forward channel or down-link (from the base station to the mobile stations) to facilitate coherent detection in the mobile station receivers. However, the use of such a common pilot signal is not feasible in the reverse channel or up-link.
An article by F. Ling entitled "Coherent Detection With Reference-Symbol Based Channel Estimation For Direct Sequence CDMA Uplink Communications", IEEE Vehicular Technical Conference, VTC '93, pages 400-403, May 1993, proposes to insert reference (pilot) symbols at a relatively high rate of 1 in every 6 symbols in the reverse channel to facilitate coherent detection. However, this technique is not compatible with an IS-95 system.
Accordingly, only non-coherent detection techniques have been presumed to be practical for the reverse channel in an IS-95 system.
An approach for non-coherent demodulation of PSK (Phase Shift Keying) modulated signals, with a performance that tends towards that of coherent demodulation, is described in an article by H. Leib et al. entitled "The Phase Of A Vector Perturbed By Gaussian Noise And Differentially Coherent Receivers", IEEE Transactions on Information Theory, Vol. 34, No. 6, pages 1491-1501, November 1988. This approach is generalized in an article by H. Leib entitled "Data-Aided Noncoherent Demodulation Of DPSK", IEEE Transactions on Communications, Vol. 43, No. 2/3/4, pages 722-725, dated February/March/April 1995 and published on or about April 24, 1995. However, these approaches do not relate to orthogonally modulated signals.
An object of this invention is to provide methods of signal demodulation and diversity combining in a communications system using orthogonal modulation, especially for the reverse channel in an IS-95 system, and to apparatus for carrying out the methods.